Two-Colour Pump-Probe Spectroscopy

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The technique

In a pump-probe experiment, a pump pulse excites a sample and induces changes that are measured using a subsequent probe pulse. By varying the time between pump and probe pulses, one can retrieve the recovery time scales of the sample. A concrete example is the measurement of the transmittance through a sample. The pump pulse modifies the transmittance of the sample. For small time difference between the pump and probe, the transmittance of the probe will be low and will increase with increasing time delay. By measuring the transmittance as a function of the delay, it is possible to observe the transmittance recovery timescale of the sample. Knowledge of the relaxation of the complete optical gain and index after a sub-picosecond light pulse is essential to assess the potential of current quantum dot materials for photonic systems applications. Pump-probe techniques provide direct, time domain, measurement of gain and refractive index nonlinearities in optical waveguides with sub-picosecond resolution.

Pump-Probe Experimental Setup
Fig. 1: Schematic diagram of the experimental setup for heterodyne pump-probe spectroscopy.
Pump-Probe Movie
Fig. 2: Animation showing how the gain recovery following the pump pulse (red) is measured by varying the delay between the pump (red) and probe (blue) pulses.

Pump-probe spectroscopy is necessary since the timescales of carrier processes in materials (typically tens of pico-seconds) are usually much faster than the bandwidth of conventional detectors. Pump-probe experiments are commonly carried out using picosecond or femtosecond lasers that emit pulse trains. In such a case, the light emitted by a laser is separated into pump and probe beams that reach the sample at different times and, after propagation through the sample, the two beams are again separated in order to analyse only the probe beam. The separation of the pump and probe beams after propagation through the sample can be achieved if the two beams have orthogonal polarisation or different frequencies. This latter solution has led to the development of heterodyne pump-probe spectroscopy.

The heterodyne pump-probe technique, in which pump and probe pulses are distinguished by inducing a small frequency shift between them, was demonstrated for the first time by K. Hall in the mid 1990’s. The technique allows separate extraction of the gain and refractive index dynamics in the waveguide and works for orthogonally as well as parallel polarised pump and probe pulses. In the experiment, a high intensity short pump pulse is used to excite the investigated sample by modifying various carrier populations. It leads to changes in the optical properties of the sample, which can be measured as a change of the transmission (gain) or phase (refractive index) of the low intensity short probe pulse.

In order to follow the gain and refractive index recoveries, the delay between pump and probe pulses is changed and the time resolution of the measurement is determined mainly by the duration of the pulses. Both pump and probe pulses follow the reference pulse which is used for detection purposes. The pump-probe technique provides gain and refractive index recovery at true operating conditions which are essential for the assessment of the optical devices used in ultra fast signal processing.

Phase measurement

A change in the amplitude of the reference-probe beating signal, caused by pump induced changes in the probe transmission, is measured using the R component (magnitude) of the high frequency lock in amplifier. Similarly a pump induced change in the refractive index, will change the phase of the probe beam with respect to the reference beam. This change is measured directly by the in-phase (X) and out-of-phase (Y) components of the high frequency lock-in amplifier.

In order to perform background free measurements, the pump beam is mechanically chopped at low frequency and the analog output of the high frequency lock-in is connected to a low frequency lock-in amplifier. The low frequency lock-in locks onto this signal, thereby eliminating all changes in the signal which do not oscillate at the same frequency as the chopper.

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Our research

Pump-probe experiment SOA
Pump-Probe Process Diagram
Fig. 3:  Schematic diagram of the processes involved in pump-probe spectroscopy of InAs/GaAs quantum dots.

Quantum dot (QD) photonic materials have attracted much study in recent years as they have the potential to deliver the stability and coherence of atomic sources within a compact and efficient semiconductor device. Characteristics such as reduced sensitivity to optical feedback and reduced alpha parameter have made such materials attractive as laser sources. Also, the suppression of pattern effects in QD semiconductor optical amplifiers (SOAs) shows promise for high speed applications. The understanding of the high speed carrier dynamics of these materials is crucial for their optimization and exploitation. To address this issue, time resolved spectroscopy has been used to investigate the fundamental carrier decay time scales of SOA structures and determine their suitability for high speed applications.

Digital optical techniques and components are vitally important in present day ultrafast communications networks. Recently, there has been much interest in using quantum dot semiconductor optical amplifiers (QD-SOAs) as non-linear elements for high speed optical switching. The basic element in high speed digital processing is a fast gating device, such as the SOA, where one optical signal controls a gate that switches a second optical signal. To evaluate the suitability of QD-SOAs for different ultrafast systems applications, a detailed understanding of the gain and refractive index recovery is essential.

Time-resolved spectroscopy has long been used to investigate the fundamental carrier decay timescales of novel semiconductor optical amplifier structures to determine their suitability for high-speed applications. We have used a heterodyne pump-probe method to determine the dependence of the capture time of InAs/GaAs quantum dots on bias current. The measured power law relationship between these two quantities was in good agreement with a model which assumed Auger dominated capture and recombination processes. This finding is important for future optical information processing applications since it implies that the recovery time of the ground state gain should be extremely fast for sufficiently high current levels.

By examining the absorption recovery dynamics, we demonstrated that the hole redistribution processes are extremely fast (1 ps) due to the effective mass asymmetry in InAs QDs. In addition, we have analyzed the gain dynamics far above transparency and found that the ES-GS relaxation is also a fast process, while Auger mediated electron capture to the QD constitutes the main limiting time scale in these devices. We have developed a rate equation model which is in good agreement with experiment. Such results are extremely relevant for the engineering of the next generation of high speed optical components such as regenerators and logic gates as QDs may offer opportunities due to their unique carrier dynamics.

[1] I. O' Driscoll, T. Piwonski, C. F. Schleussner, et al., Appl. Phys. Lett, 91, 071111 (2007)

Gain plots Gain and phase
Fig. 4: Gain regime in InAs/GaAs QDs, showing the recovery of the probe transmission at 200mA. Single colour pump GS probe GS (SCGS-green), Single colour pump ES probe ES (SCES-black), Two colour pump ES probe GS (TCGS-blue), Two colour pump GS probe ES (TCES-red). Fig. 5: Gain and phase recovery in QD semiconductor optical amplifier.

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Dr. Tomasz Piwonski

Tel: +353 21 490 4887

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